Darwin uses several examples to make the argument that, with the assistance of human breeders, animal species are capable of "rapid improvement or modification." He argues that there is clear evidence, visible to all, that animals can change very quickly. For instance, skilled breeders have, "even within a single lifetime, modified to a large extent some breeds of cattle and sheep." Sir John Sebright, a pigeon breeder whom Darwin refers to as "most skilful," bragged that "he would produce any given feather in three years, but it would take him six years to obtain head and beak."

The implication is clear. If, in these short lengths of time, animals can change so greatly in appearance, couldn't they, over longer periods, evolve into separate species? Yes, Darwin says. Continuing, he writes that "at least a score of pigeons might be chosen, which if shown to an ornithologist, and he were told that they were wild birds, would certainly, I think, be ranked by him as well-defined species." Having made this statement, he makes many arguments for the descent of all these varieties of pigeons from one species, the rock-pigeon.

In the Darwin's Ghost version of this chapter, Steve Jones makes many of the same arguments as Darwin, but uses dogs as his example, as they are now much more popular than pigeons. Picking up Darwin's arguments about pigeons, Jones argues that one has to admit that dogs have diverged into several breeds over time, as the "sole alternative… is to believe that each breed descends from a separate wild ancestor, now extinct with no token of its passing." He also gives a short history of dogs, and notes that writers and dog shows have recognized greater numbers of breeds over time. "A 1570 book recognized just seventeen," he continues. "By 1850, one writer recognized forty breeds, and the Crufts show of 1890 had two hundred and twenty classes."

The many pieces of evidence gathered by Darwin, Jones, and others seem to make a convincing argument that variations don't just happen; something drives them. In domestication, this is usually the will of the owner, exercised either by careful breeding for certain traits, or by less conscious preference of certain traits in animals. Some people, however, have trouble seeing this seemingly obvious phenomenon. "I have seen it gravely remarked," Darwin wrote, "that it was most fortunate that the strawberry began to vary just when gardeners began to attend closely to this plant."